The subject matter disclosed herein concerns improvements to turbomachines. More specifically, the subject matter disclosed herein concerns improvements to scrolls or volutes for turbomachines, such as centrifugal compressors.
Compressors are used in a wide variety of applications in industry and also in the aviation sector.
A compressor usually comprises one or more sequentially arranged stages, each comprised of a rotating impeller and a diffuser. Gas flows through the impeller and is accelerated by the impeller rotation. The kinetic energy of the gas is at least partially converted into pressure energy in the diffuser. The gas exiting the diffuser is returned to the inlet of the subsequent impeller. The gas exiting the diffuser of the last impeller is delivered to a volute or scroll, wherein the compressed gas is collected and conveyed to the outlet of the compressor.
FIG. 1 illustrates a section along the rotation axis A-A of a multistage centrifugal compressor 100 of the current art. The compressor comprises a casing 101 wherein a rotor 103 is rotatingly housed. The rotor 103 comprises a shaft 105 whereon impellers 107A-107G are mounted. Each impeller 107A-107G is in turn combined with a diffuser 109A-109G. Return channels 111A-111F are arranged downstream each diffuser 109A-109F. Each return channel 111A-111F returns the partially compressed gas from the upstream diffuser 109 to the inlet of the downstream impeller 107.
The gas exiting the last impeller 107G and the last diffuser 109G is collected in a volute or scroll 113, wherefrom the gas is delivered to a compressor outlet (not shown).
Compressors are designed to operate at or around a design point, where the maximum efficiency is achieved. When the operating conditions change the compressor still operates, for example processing a smaller or larger amount of gas, but the overall efficiency of the compressor decreases. The loss of efficiency when operating at a distance from the designed point is caused by various factors partly linked to the modification of the velocity vectors of the gas flow.
Losses are caused in particular also in the volute or scroll 113, when the gas flow rate exiting the diffuser 109G is different from the designed rate. The gas exiting the impeller 107G has a velocity vector with a tangential component and a radial component. The radial component contributes to the actual advancement of the gas in the diffuser 109G, while the tangential component causes losses. The reverse happens in the volute or scroll 113, wherein the tangential component contributes to the advancement of the gas through the scroll towards the outlet, while the axial component of the gas velocity generates vorticity and consequent losses in the flow.
There is a need for improvements to the scroll or volute of a turbomachine such as a centrifugal compressor, in order to reduce the dependency of the efficiency of the scroll of the operating conditions of the turbomachine and in particular to reduce losses when the turbomachine operates far from the designed point.